Background: Ginkgo biloba extract (GBE) is an alcohol extract of leaves from the Ginkgo biloba tree. The extract is available in form of tablets or capsules and its main medical use to improve memory and brain function. In spite of widespread human exposure to relatively high doses over potentially long periods of time, there is few studies regarding the toxicity and carcinogenicity associated with GBE. Aim: to study the potential toxicity of GBE on thyroid gland. Methods: solutions containing GBE in corn oil were administered by peroral intubationto male and female rats five times a week for three months. Groups of 20 rats received 40 mg/kg of GBE, and another group received 500 mg/kg five times a week. Another group of rats given solutions of corn oil with no chemical added. Similar group of animal of rats were given nothing except food and water and served as the blank control groups. Thyroid hormones including thyroid stimulating hormone, total triiodothyronine (T₃), and total thyroxine (T₄) were measured during the study. At the end of the study, thyroid tissues were examined for every animal by light and electron microscope.
Results:  In the small dose group ,thyroid gland showed disorganized follicles of varying diameters with little amount of the colloid in some follicles while others demonstrated absent colloid with desquamated epithelial cells in their lumens. In the high dose group, thyroid gland was composed of very small follicles. Some follicles had no apparent lumina. Follicular cells were found in more than one layer (adenoma) with Infiltration of interfollicular spaces by fatty cells.
Conclusions: GBE is a complex mixture that induces only pathological changes in rat thyroid gland